The invention concerns also the discs obtained by this method, which are meant to be used for long term archival purposes, or more generally supports the utilization conditions of which have to be highly reliable during lengthy periods of time during which the disc can be read as many times as necessary.
Various manufacturing methods of optical read discs of high quality are already known, notably by providing a polished glass disc previously coated with a thin layer or film of a photosensitive resin, in which the coded information which is to be read later is provided in the form of a latent image created by a laser beam modulated by a control assembly as a function of the signal to be encoded. The development and elimination of the resin in the zones where the image has been recorded leave in the film a succession of micro-holes allowing carrying out afterwards, between the portions of the remaining layer, the engraving of the disc in these zones. The resin is then eliminated, notably by oxygen plasma, the engraved face of the disc being finally coated with a thin reflecting metallic layer by vacuum evaporizing, said layer being in turn covered by a final protective layer. The disc thus obtained, which forms a unique original specimen, can be used as such and has, due to the inorganic nature of the materials used, a satisfactory length of life.
As a variant, when one wishes to produce a limited number of specimens of the disc, one can make directly a reproduction mask by using a technique which is substantially similar, then use this mask for making the required number of discs by photolithoengraving.
In one or the other case, the product obtained exhibits a large stability of the engraving and a satisfactory behavior to temperature and moisture, allowing in particular to meet the long term reliability and behavior objectives which are necessary for forming an archival storage medium of quality. However, the product has the disadvantage of being costly, notably when a single disc has to be made or when such a disc has to be manufactured in a very small quantity. In particular, for the person possessing the information to be recorded, the manufacture of a disc of this type requires generally to resort to a specialized laboratory, by handing over to the latter a magnetic tape or similar support allowing transcribing the coded information via a computer controlling the beam recording the sensitive layer. It should be noted that the latter has a smooth surface and does not include a priori any preformatting of the zones to be recorded, and finally that it is directly in contact with the outer environment. Under such conditions, the manufacture of the disc requires using complex and costly installations, as well as regards the necessary equipment as the means for preserving the surface of the disc from the ambient pollution, notably by making it necessary to operate in so-called "blank" premises, with an atmosphere strictly controlled. These elements have a definite incidence on the cost price of the disc, particularly if the required number of such a disc is small, which is generally the case due to the nature of the product in consideration.
In order to reduce the cost, simpler recording assemblies have already been proposed, which can be used by the owner or user himself of the information to be recorded, the engraving being made on "worm" type discs where the support is coated with a thermosensitive layer adapted for recording the image representing the information, this layer being protected outside by an appropriate film, notably composed of a polymerized resin. The recording is made from the opposite face of the transparent support which has been advantageously preformatted to ensure the guiding of the laser beam coming from the recording assembly, which simplifies quite appreciably the design of the latter and therefore the capital costs involved. Moreover, with the aforementioned "worm" type disc, the sensitive layer which receives the recording is permanently protected, notably against the outer environment, by a protective layer, thereby offering the possibility of avoiding having to operate the assembly in a protected environment, since the recording operations can be carried out in a normal atmosphere.
In such a type of disc, the recording of the thermosensitive resin layer is most often achieved by thermal ablation, that is by the formation of holes in this layer, through the glass substrate or support. Each hole corresponds to an elementary signal so that, when reading the disc by a light beam, the diffraction of the latter by the discontinuity created in register with each hole of the layer returns the original signal, each hole being equivalent to an engraved microcup such as that made in a standard disc. A stabilization layer can be advantageously provided between the support and the thermosensitive layer to enhance the recording precision. Such "worm" discs are notably the object of various prior art patents, such as U.S. Pat. No. 4,414,273 and EP-A-0 130 026.
But these discs do not possess a sufficient long term reliability for certain applications, particularly for providing archival media, and do not offer a convenient guarantee of a perfect reproduction of the signal recorded after a period of a few years. The applications are therefore limited, notably to the manufacture of proof discs, the use of traditional methods with the mentioned disadvantages being indispensable each time it is necessary to have a disc with a reliability guaranteed for a long period of time.